Why has one of my new birds STOPPED laying?

[Chasingsquirrels]

I got three new birds in late February, to add to the one I had left.
By late March they started laying, with all 4 laying most days by early April.
Happy egg days...
Then, just over a week ago my White Leghorn - who had been laying every day for at least two weeks, stopped laying, and hasn't laid since.

I wondered if she was laying elsewhere, but can't find a nest. So I kept her in the small run (they have an enclosure about 6m x 5m) for a day to make sure - no egg.

I've now let them into the back garden so that they are on grass (just on dirt in their enclosure), I think this is the third day - still no egg.

The other three are all laying, mostly every day but maybe missing once a week, if that.

Any ideas as to what could be wrong, and more importantly how to fix it!

She looks ok, although she is a lot thinner than the others - but no thinner than she was two weeks ago.

Is she maybe moulting? That would make her look thinner too. My white leghorn is the most regular layer I have, but I seem to remember her stopping quite suddenly when she moulted - it usually happens in the autumn, but it can be in spring too.

Thanks. She doesn't seem to be, and she is new so wouldn't moult yet would she? I've never had ones moult within a couple of months of getting them as point of lay before.
It just doesn't make any sense as all the reasons I think of are right.

Ok, so you've checked for any eggs inside? Crop ok? Does her breath smell?

There may not be anything wrong, but she might be at her 'tipping point' - enough energy to lay eggs but only just... some hens will lay half-formed eggs. others stop & start.
My leghorn laid a HUGE egg last week - double yoker twice the volume of another hen's egg, and that's meant she's had thin shells/days without laying.

I didn't realise she was so young. I think they take some time settling down - if the new birds had been getting extra light before coming to you that could have screwed up their cycle - does that make sense?

Silly question - but does she have have somewhere comfortable to lay eggs? I provided nest boxes to find out they were hated by my hens and they insisted in laying in Hemcore bags... Every so ofter the bags get too empty and the hens don't like it, so eggs are hidden behind honeysuckle & bushes!

Crop is fine, no idea about breath - she is the most flighty one and doesn't let me get near.
Pretty sure they weren't on forced light before, and they all took about a month to come into lay, then laid intermittently the first couple of weeks with a few soft shells, but since then which was a good two weeks, she was laying strong shelled eggs everyday, and now nothing for more than a week.
Oh well hopefully soon!!

Check that she's not blocked with an unlaid egg inside her, although if this does happen, it tends to be during their first lay. Hope she's OK soon.